Programmed Death 1 (PD-1), also known as CD279; gene name PDCD1; accession number NP_005009 is a cell surface receptor with a critical role in regulating the balance between stimulatory and inhibitory signals in the immune system and maintaining peripheral tolerance (Ishida, Y et al. 1992 EMBO J 11 3887; Kier, Mary E et al. 2008 Annu Rev Immunol 26 677-704; Okazaki, Taku et al. 2007 International Immunology 19 813-824). It is an inhibitory member of the immunoglobulin super-family with homology to CD28. The structure of PD-1 is a monomeric type 1 transmembrane protein, consisting of one immunoglobulin variable-like extracellular domain and a cytoplasmic domain containing an immunoreceptor tyrosine-based inhibitory motif (ITIM) and an immunoreceptor tyrosine-based switch motif (ITSM). Expression of PD-1 is inducible on T cells, B cells, natural killer (NK) cells and monocytes, for example upon lymphocyte activation via T cell receptor (TCR) or B cell receptor (BCR) signalling (Kier, Mary E et al. 2008 Annu Rev Immunol 26 677-704; Agata, Y et al 1996 Int Immunol 8 765-72). PD-1 has two known ligands, PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC, CD273), which are cell surface expressed members of the B7 family (Freeman, Gordon et al. 2000 J Exp Med 192 1027; Latchman, Y et al. 2001 Nat Immunol 2 261). Upon ligand engagement, PD-1 recruits phosphatases such as SHP-1 and SHP-2 to its intracellular tyrosine motifs which subsequently dephosphorylate effector molecules activated by TCR or BCR signalling (Chemnitz, J et al. 2004 J Immunol 173 945-954; Riley, James L 2009 Immunological Reviews 229 114-125) In this way, PD-1 transduces inhibitory signals into T and B cells only when it is engaged simultaneously with the TCR or BCR.
PD-1 has been demonstrated to down-regulate effector T cell responses via both cell-intrinsic and cell-extrinsic functional mechanisms Inhibitory signalling through PD-1 induces a state of anergy or unresponsiveness in T cells, resulting in the cells being unable to clonally expand or produce optimal levels of effector cytokines PD-1 may also induce apoptosis in T cells via its ability to inhibit survival signals from co-stimulation, which leads to reduced expression of key anti-apoptotic molecules such as Bcl-XL (Kier, Mary E et al. 2008 Annu Rev Immunol 26 677-704). In addition to these direct effects, recent publications have implicated PD-1 as being involved in the suppression of effector cells by promoting the induction and maintenance of regulatory T cells (TREG). For example, PD-L1 expressed on dendritic cells was shown to act in synergy with TGF-β to promote the induction of CD4+ FoxP3+ TREG with enhanced suppressor function (Francisco, Loise M et al. 2009 J Exp Med 206 3015-3029).
The first indication of the importance of PD-1 in peripheral tolerance and inflammatory disease came from the observation that PD-1 knockout (Pdcd1−/−) mice develop spontaneous autoimmunity. Fifty percent of Pdcd1−/− mice on a C57BL/6 background develop lupus-like glomerulonephritis and arthritis by 14 months of age and BALB/c-Pdcd1−/− mice develop a fatal dilated cardiomyopathy and production of autoantibodies against cardiac troponin I from 5 weeks onwards (Nishimura, H et al. 1999 Immunity 11 141-151; Nishimura, H et al. 2001 Science 291 319-322). Furthermore, introduction of PD-1 deficiency to the non-obese diabetic (NOD) mouse strain dramatically accelerates the onset and incidence of diabetes resulting in all NOD-Pdcd1−/− mice developing diabetes by 10 weeks of age (Wang, J et al. 2005 Proc Natl Acad Sci USA 102 11823). Additionally, using induced murine models of autoimmunity such as experimental autoimmune encephalomyelitis (EAE), or transplantation/graft-versus-host (GVHD) models, several groups have shown that blocking the PD-1-PD-L interaction exacerbates disease, further confirming the key role of PD-1 in inflammatory diseases. Importantly, evidence suggests that PD-1 has a comparable immune modulatory function in humans as mice, as polymorphisms in human PDCD1 have been associated with a range of autoimmune diseases including systemic lupus erythematosus (SLE), multiple sclerosis (MS), type I diabetes (TID), rheumatoid arthritis (RA) and Grave's disease (Okazaki, Taku et al. 2007 International Immunology 19 813-824; Prokunina, L et al. 2002 Nat Genet 32 666-669; Kroner, A et al. 2005 Ann Neurol 58 50-57; Prokunina, L et al 2004 Arthritis Rheum 50 1770).
Several therapeutic approaches to enhance PD-1 signalling and modulate inflammatory disease have been reported, using murine models of autoimmunity. One such approach tried was to generate artificial dendritic cells which over-express PD-L1. Injection of mice with antigen-loaded PD-L1-dendritic cells before or after induction of EAE by MOG peptide immunisation reduced the inflammation of the spinal cord as well as the clinical severity of the disease (Hirata, S et al. 2005 J Immunol 174 1888-1897). Another approach was to try to cure lupus-like syndrome in BXSB mice by delivering a PD-1 signal using a recombinant adenovirus expressing mouse PD-L1. Injection of this virus partially prevented the development of nephritis as shown by lower frequency of proteinuria, reduced serum anti-dsDNA Ig and better renal pathology (Ding, H et al. 2006 Clin Immunol 118 258). These results suggest that enhancing the PD-1 signal could have therapeutic benefit in human autoimmune disease. An alternative therapeutic approach more appropriate as a human drug treatment would be to use an agonistic monoclonal antibody against human PD-1. Binding of this agonistic antibody would ideally independently transduce inhibitory signals through PD-1 whilst also synergising with ongoing endogenous signals emanating from the natural PD-1-PD1-L interaction. An agonistic anti-PD-1 mAb would be predicted to modulate a range of immune cell types involved in inflammatory disease including T cells, B cells, NK cells and monocytes and would therefore have utility in the treatment of a wide range of human autoimmune or inflammatory disorders.
Whilst a number of antagonistic anti-PD-1 antibodies have been described, to date the only PD-1 agonistic antibodies described in the literature, to the best of the inventors knowledge, still also block the PD-1-PD1L and PD1-PD2L interaction. See for Example, WO2004/056875.
Accordingly there is still a need in the art for improved agonistic anti-PD-1 antibodies suitable for treating patients, in particular those which do not block the PD1-PDL1 or the PD1-PDL2 interaction.
We have now identified high affinity agonistic anti-PD-1 antibodies suitable for use in the treatment or prophylaxis of immune disorders, for example by reducing the T cell response. Non limiting examples of immune disorders that can be treated via the administration of PD-1 specific antibodies to a subject include, but are not limited to, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, Crohn's disease, systemic lupus erythematosus, type I diabetes, transplant rejection, graft-versus-host disease, hyperproliferative immune disorders, cancer and infectious diseases.